WASHINGTON – In a ruling that gives new momentum to the national push to legalize marijuana, the U.S. Justice Department said Thursday that it would not interfere with plans by the states of Washington and Colorado to sell and tax pot for recreational use beginning next year.

The department made its long-awaited announcement in a memo released to federal prosecutors. Attorney General Eric Holder had been under growing pressure to respond to the new state laws, since marijuana is still classified as an illegal drug under federal law.

While pot opponents wanted Holder to sue the states to block them from selling a banned substance, the Justice Department said that it won't bother, as long as the states police themselves well.

"Based on assurances that those states will impose an appropriately strict regulatory system, the department has informed the governors of both states that it is deferring its right to challenge their legalization laws at this time," the department said.

Advocates of legalization cheered the move, calling it a historic step toward ending marijuana prohibition across the United States. While Washington and Colorado in November were the first to approve marijuana for recreational use, 20 states — with California going first in 1996 — have approved marijuana sales for medical purposes. Others are expected to vote soon on recreational marijuana, including Alaska in 2014 and California in 2016, lobbyists predict.

"This is the most heartening news to come out of Washington in a long, long time," said Neill Franklin, executive director of Law Enforcement Against Prohibition, a group of law enforcement officials opposed to the war on drugs. "The message to the people of the other 48 states … is clear: Seize the day."

Retired Seattle police chief Norm Stamper said the move had the potential "to be a major advancement in the history of drug reform" and would put more marijuana business in the hands of legitimate businesses and away from criminal organizations. "For me, this means my fellow officers will be able to focus on their real job of preventing and solving violent crime," he said.

Opponents of legalization said the move would lead to a flood of negative consequences. "We can look forward to more drugged driving accidents, more school dropouts and poorer health outcomes as a new big marijuana industry targeting kids and minorities emerges to fuel the flames," said Patrick Kennedy, a former Democratic congressman from Rhode Island and co-founder of Project SAM (Smart Approaches to Marijuana), a national alliance that opposes legalization.

While Washington state and Colorado got the green light to proceed with their plans, the Justice Department reminded prosecutors in its memo that Congress has determined that marijuana is "a dangerous drug" and that selling and distributing it remains a serious crime.

The department said it expects the two states to prevent the distribution of pot to minors; to keep state-grown marijuana within their borders; and to prevent any revenue from going to criminal enterprises, such as gangs and cartels. The states also will be expected to crack down on drugged driving, prevent the growing of marijuana on public lands and ensure that marijuana is not allowed on federal property.

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